Composition of a selective oxidation catalyst for use in fuel cells

ABSTRACT

This invention pertains to improved formulations of platinum--molybdenum alloys for use as anode catalysts. These electrocatalysts find utility as a constituent of gas diffusion electrodes for use in fuel cells that operate at less than 180° C. or in applications whereupon hydrogen is oxidized in the presence of carbon monoxide or other platinum inhibiting substances. The new formulations derive unexpected activity through creating highly dispersed alloy particles of up to approximately 300 Å on carbon supports. The desired activity is achieved by carefully controlling the platinum to molybdenum ratio during preparation and judiciously selecting a proper loading of alloy on the carbon support.

This application claims priority over U.S. Provisional application Ser.No. 60/081,725, filed Apr. 14, 1998.

STATE OF THE ART

As mankind expands his presence and activity throughout the world, he isoften limited by the availability of electrical energy to support hisendeavors. Fuel Cells offer one solution to this dilemma by directlyderiving electricity from chemical feedstocks such as oxygen andhydrogen. The Fuel Cell approach also offers the potential to reducepollution problems inherent in direct combustion technology.Applications for Fuel Cells include power for vehicular traction,stationary power for home and industry, and power supplies for marineuse. However, pure hydrogen fuel is not always available, and thedevelopment of distribution means for hydrogen is uncertain.

In order for the Fuel Cell technology to realize the potential as ageneric energy source, flexibility in the choice of fuel is needed.Large-scale technology such as Solid Oxide Fuel Cells (SOFC) andPhosphoric Acid Fuel Cells (PAFC) achieve some feed flexibility byoperating at high temperatures, and thus "burn" some of the anodecontaminants that typically result from deriving hydrogen fromcarbon-containing feedstocks such as methane or propane. Both PAFC andSOFC technology are not amenable to the smaller scales (approximately<200 Kwatts) envisioned for automotive, and other applications citedabove.

The Polymer Electrolyte Membrane Fuel Cell (PEMFC) is often cited as theappropriate energy source for applications requiring less than around200 kWatts, and also for devices needing as little as a few hundredwatts. This class of fuel cell operates at less than 180° C., and moretypically around 70° C. due to the limitations in the stability of thepolymer electrolyte membrane. There is great enthusiasm behind the PEMFCapproach based on this system's lack of liquid electrolyte, ease ofconstruction, and high specific power as a function of volume or mass.

In order to impart some fuel flexibility for the PEMFC, an additionalfuel-reforming component is needed. The "reformer" convertshydrogen-containing substances such as methane, propane, methanol,ethanol, and gasoline into hydrogen gas, carbon monoxide, and carbondioxide through either a steam reformation reaction, partial oxidation,or a combination of both. Reformer technology has now advanced to thestate whereby commercially units are available. For example, a newlyformed company Epyx (Acorn Park, Cambridge, Mass.) offers a fuelprocessor that converts gasoline into hydrogen. Johnson Matthey PLC(London, UK) offers a HotSpot™ fuel processor that converts methanolusing a combination of steam reforming and partial oxidation. For boththese technologies, the untreated output is hydrogen and approximately1-2% carbon monoxide. Through additional clean-up, the carbon monoxidecan be reduced to around 50 ppm or less.

Platinum has long been acknowledged as the best anode catalyst forhydrogen. Early fuel cells employed particles of platinum black mixedwith a binder as a component in gas diffusion electrodes. The use ofplatinum black for hydrogen has been largely supplemented by the highlydisperse and very active catalysts created by the methods similar tothat found in Petrow and Allen, U.S. Pat. No. 4,082,699. This patentteaches the use of using finely divided carbon particles such as carbonblack as the substrate for small (tens of angstroms) particles of thenoble metal. Thus called a "supported" catalyst, this methodology hasshown superior performance and utilization of the catalyst inelectrochemical applications. However, while supported platinumcatalysts have demonstrated high activity for hydrogen oxidation, thisproclivity for facile kinetics is severely retarded with carbon monoxideconcentrations of only a few ppm.

Thus, with a fuel processor technology producing hydrogen streamscontaining around 50 ppm CO and platinum-based gas diffusion anodesbeing poisoned slowly with as little as 1 ppm, there is a clear need fora CO tolerant catalyst. The current state-of-the-art CO tolerantelectrocatalyst is a platinum ruthenium bimetallic alloy (Pt:Ru) and isavailable commercially in supported form (E-TEK, Inc., Natick, Mass.).The mechanism for CO tolerance is believed to involve the nucleation ofoxygen containing species (OH_(ads)) on the ruthenium site such thatplatinum-adsorbed CO can participate in a bimolecular reaction with theactivated oxygen thereby freeing the platinum site for hydrogenoxidation. However, the ruthenium site is also prone to poisoning by COat higher concentrations of CO, and the important nucleation of oxygencontaining species is then inhibited (H. A. Gasteiger, N. M. Markovic,and P. N. Ross; J. Physical Chemistry, Vol. 99, No. 22, 1995, p 8945).Although Pt:Ru has been optimized and thoroughly studied to show that analloy composed of Pt:Ru in the atomic ratio of 1:1 yields the besttolerance to CO, this bimetallic catalyst functions only at around 10ppm CO or less because of the eventual poisoning of the ruthenium site.

A recent monograph reviewing bimetallic electrocatalysts has summarizedseveral important facts in the preparation and activity ofelectrocatalysts (P. N. Ross: "The Science of Electrocatalysis onBimetallic Surfaces," in Frontiers in Electrochemistry, Vol. 4, J.Lipowski and P. N. Ross Jr., Wiley-Interscience, New York, N.Y., 1997).The activity of a bimetallic catalyst is dependent on electronic andstructural effects. Electronic properties are determined by the electronconfiguration of the alloying elements while structural properties aredetermined by both the selection of alloying elements and the method ofpreparation of the alloy itself. This last observation is important inthe design of CO tolerant catalysts. For example, a Pt:Ru alloy preparedby sputtering a bulk alloy, annealing a bulk alloy, or depositing asubmonolayer of ruthenium on platinum all yield fundamentally differentcatalytic properties (P. N. Ross, p 19). The precept that alloyformation methodology influences catalyst function follows from thecreation of three zones in every bimetallic catalyst: metal "A", metal"B", and an intermixed zone "A-B". The distribution of these zonesdetermines activity.

Another important property noted by Ross in the monograph is that thephenomenon of surface segregation in bimetallic alloys has often beenneglected. Surface segregation is the enrichment of one element at thesurface relative to the bulk, and in our case would be dominated byplatinum in an alloy of 4d elements with the exception of silver and tin(Ross, p. 51).

In summary, there is ample evidence to show that electrocatalysts candiffer in their activity due to preparation methods. Another differencearises from dissimilarities between the bulk and surface compositions ofthe alloy. For these two reasons, we expect even greater contrasts tooccur between bimetallic alloys prepared as bulk metals compared toalloys prepared as very small (10 to 300 Å) supported particles.

Molybdenum has been observed to play a catalytic role in the oxidationof small organic molecules otherwise known as "C1" molecules (todesignate one carbon atom). As early as 1965, a molybdenum platinumblack complex was implicated in the catalytic oxidation of formaldehydeand methanol in sulfuric acid (J. A. Shropshire; Journal of theElectrochemical Society, vol. 112, 1965, p. 465). Although themolybdenum was added as a soluble salt, it was reduced and depositedonto the platinum black electrode. Later on, several others took note ofthis property of molybdenum and tried to intentionally create platinumalloys. H. Kita et al. confirm that a platinum molybdenum complex formedthrough reduction of the metal salt onto the surface of the platinumfoil electrode can catalyze methanol oxidation (H. Kita et al.; J.Electroanalytical Chemistry, vol. 248, 1988, p.181). H. Kita extendedthis work to creating a membrane electrode assembly (MEA) of chemicallydeposited platinum and molybdenum on Nafion, to be used in a PEMFC. Asbefore, the fuel here is methanol (H. Kita et al.; Electrochemistry inTransition, Oliver Murphy et al., Eds., Plenum Press, New York, 1993, p.619). These are both examples of forming an alloy through deposition ofa submonolayer of molybdenum onto platinum, although no high surfacearea support is used.

Masahiro Watanabe discloses the use of vacuum sputtering to form analloy of Ni, Co, Mn or Au with Pt, Pd, or Ru. The object of this patentis to provide a CO tolerant anode catalyst for the PEMFC (MasahiroWatanabe: Japan Patent Application No. H6-225840, Aug. 27, 1994).Although this patent directs towards a preferred alloy consisting of Ptwith Ni, Co, Mn, or Au, a comparison example of Pt with Mo is shownwhereby sustained currents for hydrogen oxidation in the presence of COdissolved in sulfuric acid are recorded. The example employs a rotatingdisk electrode coated with an alloy formed by simultaneous argonsputtering under reduced pressure. While the patent emphasizes the useof sputter coating, some mention is made to carbon supported alloysprepared by the usual thermal decomposition methods. However, there isno description or teaching as to how the properties achieved in asputter-coated alloy could be obtained by thermal decomposition ontocarbon black.

A recent publication indicates the potential for Pt:Mo as a CO tolerantcatalyst superior to Pt:Ru (B. N. Grgur et al.; Journal of PhysicalChemistry (B), vol. 101, no. 20, 1997, p. 3910). In this paper, a sampleof Pt₇₅ Mo₂₅ alloy is prepared as a bulk crystal by arc melting of thepure elements in an argon atmosphere and homogenizing with a heattreatment. The authors show that the resulting boule possessed a uniformmetal alloy composition from the interior bulk to the surface. This wellcharacterized surface is formed into a rotating electrode disk and showsoxidation of hydrogen in a mixed gas of H₂ /CO. The authors put forthevidence that the molybdenum may participate in a greater rate of COoxidation compared to the ruthenium. Furthermore, the authors point outthat ruthenium and platinum do not differ much in that they both absorbH₂ and CO, possess quasireversible OH_(ads) states, and areelectrocatalysts for H₂ and CO: the alloying process does not produce afundamental change in the properties of either metal. On the other hand,molybdenum is significantly different than platinum, and formation ofthe alloy produces a material with substantial differences in theintrinsic chemical properties. While the authors relate a surface withunexpected catalytic properties, there is no mention of how one couldtranslate the properties discovered in this bulk alloy to the highlydisperse carbon supported catalysts employed in gas diffusionelectrodes.

There has been some effort in the patent literature to create thesupported Pt:Mo alloy on carbon blacks. Landsman et al. in U.S. Pat. No.4,316,944 describe a method to form noble metal chromium alloys oncarbon black for eventual incorporation into a cathode of a fuel cell.In this case, the inventors were seeking superior oxygen reductioncatalysts for use with PAFC. They make use of a powder ofalready-dispersed platinum on metal and a solution of ammonium chromate.The addition of dilute hydrochloric acid was added to cause theadsorption of the chromium species on the supported catalyst. Heattreatment in nitrogen was used to form the platinum chromium alloy.Although Pt:Mo appears in a table of results as a cathode catalyst, nodetails are given to its preparation, metal:metal ratio, or metal oncarbon weight loading.

Thus, there is a need to show a method of preparation and formulationrequirements that preserve the unexpected CO tolerant properties ofPt:Mo on carbon black supports that would then allow this alloy to bereadily incorporated into gas diffusion electrodes or membrane electrodeassemblies (MEAs).

DESCRIPTION OF THE INVENTION

It is an object of this invention to provide an improved high-surfacearea formulation of platinum:molybdenum on a carbon support whereby: thebulk atomic ratio of Pt:Mo is between 99:1 and 1:1, preferably between3:1 and 5:1, and more preferably 4:1; and the metal loading of alloy oncarbon support is between 1% and 80% total metal on carbon, preferablybetween 20% and 40% It is a further object of this invention to providean anode catalyst for a fuel cell whereby hydrogen can be oxidized inthe presence of carbon monoxide.

It is also an object of this invention to provide a method ofmanufacturing supported platinum molybdenum alloy with highly desirablesurface activity. It is a final object of this invention to provide ananode catalyst with high activity for the direct oxidation of smallorganic molecules such as methanol.

Amongst the aforementioned methods of forming a bimetallic alloy, wehave found that a combination of deposition and bulk annealing forms themost potent form of the alloy. As has been previously established, theprecipitation of metal salts onto carbon black supports can yield highlydisperse formulations of metal.

For example, through the teachings of Petrow and Allen, a complex ofplatinum sulfite acid produces extremely small and well-dispersedparticles of platinum on carbon black. The Table below illustrates therelationship between weight loading on carbon black (here Vulcan XC-72),the resulting average platinum cystallite size, and the effectiveplatinum surface area.

                  TABLE 1                                                         ______________________________________                                        Weight Loading of Platinum as a function of crystallite size and surface      area.                                                                         Catalyst loading on                                                           Vulcan XC-72, in                                                                            Average Pt Particle Size                                                                     Pt Surface Area                                  % (wt/wt)     Å          m.sup.2 /g                                       ______________________________________                                        10            20             140                                              20            25             112                                              30            32             88                                               40            39             72                                               60            88             32                                               80            250            11                                               ______________________________________                                         Reproduced from ETEK, Inc. Gas Diffusion Electrodes and Catalyst              Materials, Catalog, 1998, p 15.                                          

While there are clear trends with regards to particle size and effectivesurface area, it is important to note that the specific activity of thecatalyst follows a trend as well. As reviewed by Markovic, Gasteiger,and Ross in The Journal of the Electrochemical Society, Vol. 144, No. 5,May 1997, p 1591, the oxygen reduction rate and hence activity ofplatinum can be highly sensitive to the type and abundance of crystalface (111, 100, and 110). Furthermore, Markovic et al. point that theplatinum crystallite size controls the relative abundance of the variousface geometries. Since the activity of a CO tolerant alloy depends onthe final structure of the alloy crystal, control of metal loading,particle size, and distribution of particle size all play a vital roleas well as the actual method of alloy formation.

In one preferred embodiment, manufacture of platinum-molybdenum alloysbegins by first depositing platinum on a carbon black. Colloidalparticles of Pt oxide are deposited on a carbon support from an aqueoussolution of a platinum precursor containing the support material. Inorder to form a colloid, the platinum containing species can besubjected to an oxidizing agent or the solution can be simplyevaporated. Although Pt sulfite acid is the preferred choice for theprecursor, chloroplatinic acid could alternatively be used. In a secondstep, discrete particles of Mo oxide are deposited on the Pt oxidecontaining carbon support by adsorption of colloidal Mo oxide or Moblue, formed in situ by mild reduction of a solution containing a Moprecursor, for instance an ammonium molybdate solution or a solutioncontaining Mo with alkali hydroxide. Several chemical reducing agentsmay be employed as well known to one skilled in the art, for examplehydrazine, formic acid, formaldehyde, oxalic acid, or metals having asufficiently low potential such as molybdenum and zinc: another methodfor reducing the Mo containing solution consists in feeding saidsolution to an electrochemical cell, applying direct current thereto andreducing the Mo precursor at the cathode. After drying, the catalyst isfirst subjected to a reducing atmosphere between 500 and 900° C., andthen alloyed at higher temperature (for instance at 900 to 1200° C.) inthe same reducing atmosphere or in an inert one: in one preferredembodiment, it may be reduced at 500-800° C. in H₂ gas, then heattreated at 800-1200° C. in Ar gas to form the alloy phase of Pt and Mo.In another preferred embodiment, reduction and alloying are bothperformed in a H₂ environment between 500 and 1200° C., either in asingle or in two subsequent temperature steps. This general method isapplicable to preparations of Pt:Mo alloys supported on amorphous and/orgraphitic carbon materials with a ratio of Mo alloyed with Pt from 1 to50 atomic % and a total metal loading on the carbon support from 1-90%.It is however preferred that the total metal loading be comprisedbetween 10 and 40%. This method produces a carbon supported Pt:Mo alloycatalyst with a metal particle size of approximately 300 Å or less.Other methods for preparing carbon supported Pt:Mo alloys of the samecharacteristics will be given in detail in the following examples.

Catalysts produced in this manner are readily incorporated into gasdiffusion electrodes For example Pt:Mo catalysts thus prepared can beincorporated into structures similar to the commercially available ELAT®(E-TEK, Inc., Natick, Mass.). Here, a carbon cloth serves as the web. Alayer of Shawinigan Acetylene Black (SAB) mixed withpolytetrafluoroethylene binder (e.g. Teflon® commercialized by DuPont,Wilmington, Del.) serves as the wetproofing layer on each side of theweb. Finally, layers of carbon black such as Vulcan XC-72 with the alloyPt:Mo are coated onto one side of the assembly: preferably, the specificloading of metal with respect to the active area is comprised between0.1 and 5 mg/cm². After the final coat, the assembly may be sintered inair at a temperature sufficient to cause the binder to flow, typically300-350° C. Allen et al. in U.S. Pat. No. 4,293,396 further describe theconstruction of this type of gas diffusion electrode. Such catalysts canalso be incorporated in other gas diffusion electrode structures, forexample the electrodes in co-pending patent "Improved Structures andMethods of Manufacture for Gas Diffusion Electrodes and ElectrodeComponents" are suitable as well as described in U.S. provisionalapplication Ser. No. 60/070,342 filed Jan. 2, 1998.

These carbon-supported alloys can also be deposited onto the surface ofan ion conducting membrane such as Nafion® or Gore Select®commercialized respectively by DuPont and Gore and Associates, Elkton,Md. Wilson and references therein have described methods for suchoperations in U.S. Pat. No. 5,234,777. In general, depositing thecatalyst on the membrane through a "decal" method (see Wilson) cancreate a membrane electrode assembly, or one can apply a paint or ink ofcatalyst to the membrane, or a catalyzed gas diffusion electrode can bemechanically or heat-pressed against the membrane.

For the examples listed here, we have employed a catalyzed gas diffusionelectrode similar to that described in Allen et al. pressed against aNafion membrane. However, fuel cell tests can be highly dependent onsystem configuration. For example, the mechanical geometry one uses tomake contact between the electrode and the membrane, the flow fieldgeometry employed to feed gasses to anode and cathode, and the methodand manner of providing hydrated gasses to the cell can all affect thecell performance. In order to evaluate catalyst performance in theabsence of system variables but still as an active component of a gasdiffusion electrode, we also employ a simple three-electrode testmethod.

The three-electrode or "half cell" method fits 1 c^(m2) sample of gasdiffusion electrode into an inert holder. The gas-feed side of the gasdiffusion electrode is positioned into a plenum whereby an excess ofoxygen, air, hydrogen, or hydrogen containing levels of CO is passed atlow pressures (on the order of 10 mm of water or less). The facecontaining the catalyst (that would normally be against the membrane ofa PEMFC) is held in a 0.5M H₂ SO₄ solution at a fixed temperature. Thecounter electrode is placed directly across the working electrode, and areference electrode is held in-between the two. The fixed geometry ismaintained between the three electrodes through a specially constructedcap. A potentiostat is employed to control the potential and measure thecurrent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now better described by means of the followingexamples, which are only intended to illustrate but not limit the extentand application of this invention, and resorting to the figures,wherein;

FIG. 1 shows the potentiostated current--potential curves for samples ofStandard ELAT® with 1 mg Pt/cm², 30% Pt/C in 0.5M H₂ SO₄, atapproximately 55° C., with and without 100 ppm CO in hydrogen. Platinumfoil 3×2 cm serves as the counter electrode. A standard calomelelectrode serves as the reference. Reported potentials are corrected forIR using the current interrupt method.

FIG. 2 shows potentiostated current--potential curves for samples ofStandard ELAT™ with 1 mg Pt₅₀ :Ru₅₀ /cm², 30% Metal/C in 0.5M H₂ SO₄, atapproximately 55° C., with and without 100 ppm CO in hydrogen. Platinumfoil 3×2 cm serves as the counter electrode. A standard calomelelectrode serves as the reference. Reported potentials are corrected forIR using the current interrupt method.

FIG. 3 shows potentiostated current--potential curves for samples ofStandard ELAT™ with 1 mg Pt₇₅ :Mo₂₅ /cm², 30% Pt/C in 0.5M H₂ SO₄, atapproximately 55° C., with and without 100 ppm CO in hydrogen. Platinumfoil 3×2 cm serves as the counter electrode. A standard calomelelectrode serves as the reference. Reported potentials are corrected forIR using the current interrupt method.

FIG. 4 shows a calculation of percent loss of hydrogen current due to100 ppm CO vs. applied potential, from the tests of FIGS. 1, 2, and 3.Based on average of three or more samples. Conditions as in FIG. 1.

FIG. 5 shows a comparison of anode catalysts (Pt, Pt₅₀ Ru₅₀, Pt₉₅ :Sn₅,and Pt₇₅ :Mo₂₅) in standard ELAT™ Gas Diffusion Electrodes, 1.0 mg/cm²total metal loading using 30% Metal/C, 16 cm² Active Area, Nafion 115,Pressure for Fuel/Air--3.5/4.0 BarA, temperature 70° C., with a hydrogencontamination of 16 ppm CO.

FIG. 6 shows a comparison of anode catalysts (Pt, Pt₅₀ Ru₅₀, Pt₉₅ :Sn₅,and Pt₇₅ :Mo₂₅) in standard ELAT™ Gas Diffusion Electrodes, 1.0 mg/cm²total metal loading using 30% Metal/C, 16 cm² Active Area, Nafion 115,Pressure for Fuel/Air--3.5/4.0 BarA, temperature 70° C., with a hydrogencontamination of 100 ppm CO.

FIG. 7 shows a comparison of anode catalysts (Pt, Pt₅₀ Ru₅₀, Pt₉₅ :Sn₅,and Pt₇₅ :Mo₂₅) in standard ELAT™ Gas Diffusion Electrodes, 1.0 mg/cm²total metal loading using 30% Metal/C, 16 cm² Active Area, Nafion 115,Pressure for Fuel/Air--3.5/4.0 BarA, temperature 70° C., with a hydrogencontamination of 970 ppm CO.

FIG. 8 shows a comparison of Anode Catalysts (Pt, Pt₅₀ Ru₅₀, Pt₈₀ :Mo₂₀)in standard ELAT™ Gas Diffusion Electrodes, 1.0 mg/cm² total metalloading using 30% Metal/C, 16 cm² Active Area, Nafion 115, Pressure forFuel/Air--3.5/4.0 BarA, temperature 70° C., with a hydrogencontamination of 22 and 103 ppm CO.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

A catalyst composed of 30 wt. % alloy on Vulcan XC-72 whereby the alloyis Pt₇₅ Mo₂₅ atomic percent begins with the preparation of platinum oncarbon according to the method described by Petrow and Allen (U.S. Pat.No. 4,082,699) and is briefly summarized below.

A solution containing 38.66 ml of a 200 g/l platinum (II) sulfite acidsolution in 1.3 l of deionized H₂ O is neutralized to pH 4.0 with adilute (˜1M) NH₄ OH solution. 21 g of Vulcan XC-72 is slurried with theplatinum solution, then dispersed ultrasonically to achieve a homogenousmixture. Using a magnetic stirrer to maintain adequate mixing, 125 ml ofa 30 wt % H₂ O₂ solution is added over the course of ˜30 minutes. Theslurry is allowed to stir for 1 hour, then the pH is adjusted to 4.0with a dilute NH₄ OH solution. 75 ml of 30 wt % H₂ O₂ solution are addedover the course of ˜20 minutes and the slurry is stirred for 1 hour. ThepH of the slurry is again adjusted to 4.0, then the slurry is heated to70° C. The solids are filtered to remove the supernatant liquid, washedwith hot deionized H₂ O to remove any soluble salts, then dried at 125°C. to remove moisture.

In a second step, the platinum containing carbon catalyst prepared aboveis ground to a powder, then dispersed ultrasonically in 500 ml ofdeionized H₂ O. An ammonium molybdate solution is prepared by dissolving1.902 g of MoO₃ in ˜25 ml of concentrated NH₄ OH solution and removingthe excess ammonia by heating and stirring. This clear solution is addedto the platinum catalyst slurry under stirring and the pH is adjusted to˜1.8 with dilute H₂ SO₄. One ml of a 16 wt % N₂ H₄ solution is added toform colloidal MoO_(3-x) (molybdenum blue) in-situ and the slurryallowed to stir ˜8 hours. The addition of the reducing agent is repeatedtwice more over 24 hours to ensure a complete reaction, then the slurryis heated to 70° C. The solids are filtered to remove the supernatantliquid, washed with hot deionized H₂ O to remove any soluble salts, thendried at 125° C. to remove moisture. After grinding to a powder, thecatalyst is hydrogen reduced at 800° C. for 1 hour, then heat treated at1000° C. for 1 hour in flowing argon gas to form the alloy phase.

EXAMPLE 2

A catalyst composed of 30 wt. % alloy on Vulcan XC-72 whereby the alloyis Pt₈₀ Mo₂₀ atomic percent follows that of Example 1 except 40.07 ml ofa 200 g/l platinum (II) sulfite acid solution is substituted in thefirst step and 1.478 g of MoO₃ is substituted in the second step.

EXAMPLE 3

A catalyst composed of 30 wt. % alloy on Vulcan XC-72 whereby the alloyis Pt₈₅ Mo₁₅ atomic percent follows that of Example 1 except 40.97 ml ofa 200 g/l platinum (II) sulfite acid solution is substituted in thefirst step and 1.209 g of MoO₃ is substituted in the second step.

EXAMPLE 4

A catalyst composed of 30 wt. % alloy on Vulcan XC-72 whereby the alloyis Pt₇₅ Mo₂₅ atomic percent follows that of Example 1 except that thecolloidal solution of MoO_(3-x) (Molybdenum Blue) is preparedseparately, following the same general method described to form thisspecies in situ., then added to the platinum on carbon slurry. Thecolloidal MoO_(3-x) particles are readily adsorbed on the carbon surfaceadjacent to the deposited platinum. After filtration and drying, thealloy phase is formed as previously described.

EXAMPLE 5

A catalyst composed of 30 wt. % alloy on Vulcan XC-72 whereby the alloyis Pt₇₅ Mo₂₅ atomic percent follows that of Example 1 except that acolloidal solution of PtO_(x) is prepared by evaporation of the platinum(II) sulfite acid solution to dryness, then dissolving the solids in H₂O to form a stable colloidal dispersion. A colloidal solution ofMoO_(3-x) (Molybdenum Blue) is also prepared separately following thesame general method used to form this species in situ. The two colloidaldispersions are then added concurrently to a slurry of Vulcan XC-72 inH₂ O allowing the PtO_(x) and MoO_(3-x) species to adsorb on the carbonsurface. After filtration and drying, the alloy phase is formed aspreviously described.

Comparative Example 6

A catalyst composed of 30 wt. % platinum on Vulcan XC-72 is prepared asfollows. The platinum addition method as described in Example 1 isfollowed except now the amount of platinum (II) sulfite acid solutionadded is 45.00 ml, and after drying, the 30 wt. % platinum on Vulcancatalyst powder is H₂ reduced at 500° C. for 1/2 hour.

Comparative Example 7

A catalyst composed of 30 wt. % alloy on Vulcan XC-72 whereby the alloyis Pt₅₀ Ru₅₀ atomic percent is prepared as follows. The platinumaddition method as described in Example 1 is followed except now acombination of 29.64 ml of platinum (II) sulfite acid solution and 76.80ml of ruthenium (II) sulfite acid solution is added to 1.3 l ofdeionized H₂ O. Oxidation of the mixed sulfite acid solution with 30 wt.% H₂ O₂ results in a mixed transient colloidal solution containingdiscrete particles of PtO_(x) and RuO_(x) that adsorb simultaneously onthe carbon surface. After drying, the 30 wt. % Pt₅₀ Ru₅₀ on Vulcancatalyst powder is H₂ reduced at 230-250° C. for 1 hour to form thealloy phase.

Comparative Example 8

A catalyst composed of 30 wt. % alloy on Vulcan XC-72 whereby the alloyis Pt₉₅ Sn₅ atomic percent follows the method described in Example 1except that the amount of platinum (II) sulfite acid solution added is43.60 ml in the initial step. In the second step, 2.364 g of a stableSnO₂ colloid, commercially available from Nyacol Products Inc., Ashland,Mass., (15 wt. % SnO₂) is added to the Pt on Vulcan XC-72 catalystpowder slurry and the discrete SnO₂ particles are readily adsorbed onthe platinized carbon surface. After filtration and drying, the catalystpowder is H₂ reduced at 500° C. for 1/2 hour then heat treated at 900°C. for 1 hour under flowing argon to form the alloy phase.

The catalysts as described above are incorporated into a standard gasdiffusion electrode and subjected to small-scale testing free of systemvariables. FIGS. 1, 2, and 3 show the results of several samples of each(platinum, Pt₅₀ :Ru₅₀, and Pt₇₅ :Mo₂₅) being subjected to eitherhydrogen or hydrogen contaminated with 100 ppm CO. These are considered"driven" cells in as much as the potentiostat applies a potential, thefeedgas is consumed, and current is developed. In FIG. 1 one readilynotes the devastating effects of CO on pure supported platinum:currentis reduced dramatically. FIG. 2 employs the comparative example Pt₅₀:Ru₅₀ subjected to the same conditions. Here some resistance topoisoning is noted. FIG. 3 is Pt₇₅ :Mo₂₅ subjected to pure H₂ and H₂with 100 ppm CO. It is significant to note that at the higher appliedpotentials (100-200 mV vs. SCE), the current for the new alloy does notappear to plateau as in the Pt₅₀ :Ru₅₀. FIG. 4 illustrates theresilience of Pt₇₅ :Mo₂₅ more clearly. In this Figure, instead ofplotting current on the ordinate axis, the loss of current due to COpoisoning is plotted as a function of percent. Thus, the currentobtained at the electrodes in hydrogen is compared to the currentobtained at 100 ppm CO. Thus, pure platinum results in an approximately75% loss of current, while Pt₅₀ :Ru₅₀ is 50%, and Pt₇₅ :Mo₂₅ is around25%. These results illustrate an improvement over the current state ofthe art and verify that forming the platinum molybdenum alloy on acarbon black support is viable method for preparing a catalyst for highhydrogen oxidation activity in the presence of moderate levels of CO.

The next set of Figures affirms that the advances observed on the smallscale are operative within a fuel cell system. FIG. 5 shows a family ofcurves generated on a single 16 c^(m2) cell operating as an air/hydrogenfuel cell. The electrodes and catalysts represented here are prepared asdescribed above. Unlike the previous experiments, the fuel cellgenerates current and voltage proportional to the power available fromthe system and the load placed on this system. Within this family ofcurrent--potential curves two reference examples are displayed. The topcurve labeled "average Pt ELAT--H₂ data" is the case of pure hydrogenover a supported platinum catalyst, i.e., the best case. The bottomcurve of the family, labeled "Standard Pt ELAT" is the example of asupported platinum catalyst being subjected to the CO contaminatedhydrogen feed, i.e., the worst case.

Thus, FIG. 5 shows the effects of three different alloy combinationsbeing subjected to 16 ppm CO in the hydrogen. At this low level of CO,only small differences arise between the three alloys, although the Pt₇₅:Mo₂₅ appears slightly better-performing at the higher currentdensities. FIG. 6 is a plot of a similar family of curves except nowthere is 100 ppm CO contamination. At this level of CO, one notes thathigher currents and voltages are obtained from the Pt₇₅ :Mo₂₅ alloycompared to either Pt₅₀ :Ru₅₀ or Pt₉₅ :Sn₅. Similarly, the plot of FIG.7 shows the same electrodes subjected to 970 ppm CO in hydrogen with thesame result: the Pt₇₅ :Mo₂₅ alloy provides the greatest resistance to COpoisoning.

A similar alloy is prepared except now the amount of Mo is decreased toform a Pt₈₀ :Mo₂₀ alloy. FIG. 8 compares ELAT electrodes assembled withthis catalyst compared to the standard Pt₅₀ :Ru₅₀ catalyst under 22 and103 ppm CO in hydrogen. This Figure more clearly shows higher currentsbeing obtained for a fixed voltage with the Pt:Mo over Pt:Ru, especiallyover the voltage region of 0.6 to 0.7V, which is considered a moreefficient operating voltage for the fuel cell stack.

Similar experiments were performed over a range of temperatures, from60° C. to 90° C., and currents obtained at 0.7 and 0.6 V are tabulatedfor comparison. Refer to Tables 1-4 below. A column within the Tables isthe calculation of the percent decrease from pure hydrogen when thealloys are subjected to each level of carbon monoxide. In all cased,through all temperatures, the Pt₈₀ :Mo₂₀ shows a smaller percentdecrease than Pt:Ru. In all cased, the Pt₈₀ :Mo₂₀ catalyst yieldedgreater current than the commercially employed Pt:Ru. These resultsconfirm that the Pt:Mo alloy is an improved anode catalyst for a fuelcell whereby hydrogen can be oxidized in the presence of carbonmonoxide.

                  TABLE 1                                                         ______________________________________                                        Comparison of Pt4Mo to Pt:Ru at 60° C.                                 H2       22 ppm CO % decrease                                                                              103 ppm CO                                                                            % decrease                               ______________________________________                                        Current at 0.7V, 60° C.                                                Pt4:Mo                                                                              471    243       -48%    162     -66%                                   Pt:Ru 459    219       -52%    149     -68%                                   Current at 0.6V, 60° C.                                                Pt4:Mo                                                                              711    356       -50%    272     -62%                                   Pt:Ru 728    317       -56%    208     -71%                                   ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Comparison of Pt4:Mo to Pt:Ru at 70° C.                                H2       22 ppm CO % decrease                                                                              103 ppm CO                                                                            % decrease                               ______________________________________                                        Current at 0.7V, 70° C.                                                Pt4:Mo                                                                              521    330       -37%    231     -56%                                   Pt:Ru 530    304       -43%    211     -60%                                   Current at 0.6V, 70° C.                                                Pt4:Mo                                                                              790    492       -38%    365     -54%                                   Pt:Ru 831    455       -45%    304     -63%                                   ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Comparison of Pt4:Mo to Pt:Ru at 80° C.                                H2       22 ppm CO % decrease                                                                              103 ppm CO                                                                            % decrease                               ______________________________________                                        Current at 0.7V, 80° C.                                                Pt4:Mo                                                                              541    404       -25%    300     -45%                                   Pt:Ru 570    371       -35%    273     -52%                                   Current at 0.6V, 80° C.                                                Pt4:Mo                                                                              825    599       -27%    453     -45%                                   Pt:Ru 877    555       -37%    398     -55%                                   ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Comparison of Pt4:Mo to Pt:Ru at 90° C.                                H2       22 ppm CO % decrease                                                                              103 ppm CO                                                                            % decrease                               ______________________________________                                        Current at 0.7V, 90° C.                                                Pt4:Mo                                                                              578    475       -18%    386     -33%                                   Pt:Ru 573    461       -20%    343     -40%                                   Current at 0.6V, 90° C.                                                Pt4:Mo                                                                              858    689       -20%    564     -34%                                   Pt:Ru 891    694       -22%    508     -43%                                   ______________________________________                                    

Even if the invention has been described making reference to specificembodiments, it must be understood that modifications, substitutions,omissions and changes of the same are possible without departing fromthe spirit thereof and are intended to be encompassed in the appendedclaims.

We claim:
 1. A carbon black supported catalyst for use in gas diffusionelectrodes having a bulk atomic ratio of platinum to molybdenum of 3:1to 5:1.
 2. The carbon black supported catalyst of claim 1 wherein totalmetal loading on carbon black is between 10 and 90% by weight.
 3. A gasdiffusion electrode having a web, a catalyst layer, and optionally awet-proofing coating, the catalyst layer comprising between 0.1 and 5mg/cm² metal of the catalyst of claim
 1. 4. A polymer electrolytemembrane fuel cell stack fed on the anode side with a hydrogen-rich gasmixture containing at least 10 ppm CO, comprising at least one electrodeof claim
 3. 5. An ion exchange membrane coated on at least one side withthe catalyst of claim
 1. 6. The ion exchange membrane of claim 5 whereintotal metal loading is comprised between 0.1 and 5 mg/cm².
 7. A polymerelectrolyte membrane fuel cell stack fed on the anode side with ahydrogen-rich gas mixture containing at least 10 ppm CO, comprising atleast one ion exchange membrane of claim
 5. 8. The carbon blacksupported catalyst of claim 1 wherein metal loading in carbon black isbetween 10 to 40% by weight.
 9. A method of preparing a carbon blacksupported catalyst for use in gas diffusion electrodes having a bulkatomic ratio of Pt:Mo of 3:1 to 5.1 comprising:preparing a slurry ofcarbon black in a platinum containing solution, said solution furthercomprising molybdenum; adding a reducing agent; alloying the twoelements in a reducing and/or inert atmosphere at a temperature above300° C.
 10. The method of claim 9 wherein said solutions containingplatinum and molybdenum are a colloidal dispersion of platinum and acolloidal dispersion of molybdenum.
 11. The method claim 9 wherein saidalloying of the two elements is realized by subjecting the catalyst toan inert atmosphere of argon between 500 and 1200° C.
 12. The methodclaim 9 wherein said alloying of the two elements is realized bysubjecting the catalyst to an atmosphere of hydrogen between 500 and1200° C.
 13. The method claim 9 wherein said alloying of the twoelements is realized by subjecting the catalyst to a reducing atmosphereof hydrogen between 500 and 900° C., and an inert atmosphere of argonbetween 900 and 1200° C.
 14. A method of preparing a carbon blacksupported catalyst for use in gas diffusion electrodes having a bulkatomic ratio of platinum to molybdenum of 3.1 to 5:1 comprising firstabsorbing a platinum colloidal dispersion onto carbon black followed byabsorbing a colloidal dispersion of molybdenum onto carbon black, andfinally alloying the two elements in a reducing and/or inert atmosphereat a temperature above 300° C.
 15. The method of claim 14 wherein saidcolloidal dispersion of platinum is obtained upon evaporating a solutionof platinum sulfite acid to dryness.
 16. The method of claim 14 whereinsaid colloidal dispersion of platinum is obtained upon addition of anoxidizing agent to a solution containing a platinum precursor.
 17. Themethod of claim 14 wherein said colloidal dispersion of molybdenum isobtained from a solution containing molybdenum species in alkalihydroxides or ammonia by adding a reducing agent.
 18. The method ofclaim 17 wherein said molybdenum species are selected from the groupconsisting of MoO₃, molybdic acid, molybdenum blue and molybdates. 19.The method of claim 17 wherein the reducing agent is selected from thegroup consisting: hydrazine, molybdenum metal, zinc, formic acid,formaldehyde, and oxalic acid.